1. Field of the Invention
The present invention relates to an image processing apparatus which performs image processing on an input image signal, which is brightness values input at intervals of a constant period of time, based on the frequency of the brightness change of the input image signal to improve a visual effect and other effects.
2. Description of the Related Art
The image display method and apparatus described in Japanese Patent Laid-Open Publication Kokai No. 2000-4380 are configured to be able to improve the contrast of an image display without increasing the circuit size by using an infinite impulse response (IIR) filter as a low pass filter.
To improve image visual effects, such as contrast, on an image, a low pass filter comprising a finite impulse response (FIR) filter that is more stable than an IIR filter is typically used. FIG. 1 shows an image processing apparatus 1 which performs a high-frequency region intensification or enhancement process (hereinafter, referred to as “sharpening process”) to improve the visual fineness of an image by using a FIR filter as a low pass filter. In FIG. 1, a low frequency component Vlo of an input image Vin is extracted by a low pass filter (LPF) 2, and is subtracted from the input image Vin, thereby extracting a high frequency component Vh of the input image Vin. Subsequently, a gain G is multiplied to the high frequency component Vh and then a high frequency component Vhg multiplied by the gain G is added back to the input image Vin, thereby obtaining an output image Vout on which the sharpening process has been performed. When the LPF 2 is configured as, for example, a FIR filter with three taps, two pixels L and R on both sides of the center pixel C are used as evaluation pixels to calculate a brightness value of the output image. That is, the pixels L and C obtained by combining a plurality of delay elements and the pixel R are multiplied by corresponding tap coefficients aL, aC, and aR by multipliers and then summed by an adder, whereby the brightness (gradation) of the output image is calculated. The three tap coefficients aL, aC, and aR corresponding to the three pixels have respective fixed values determined from the cut-off frequency. This implies that the summation by the adder can be regarded as a process of mixing the brightness values of pixels with one another under a fixed mixture ratio.
In a method using such conventional LPFs, however, if an image includes a steep gradation change or a step input, like from zero to the maximum or vice versa between adjacent pixels, a phenomenon occurs in which the image on which a sharpening process has been performed becomes significantly different from the original one around the region where such gradation change occurs. Referring to FIG. 2, sharpening process characteristics of a conventional image processing apparatus is shown. As is shown in FIG. 2, in the brightness values of an output image signal on which a sharpening process has been performed, a large overshoot occurs around the region where a step response occurs in the brightness values of an input image signal, which are input at intervals of a predetermined period of time. This is caused by the digital signal processing of LPFs that cannot track a step input. Therefore, although the sharpening process can improve the fineness of an entire image, the problem of severely damaging the original image occurs as an adverse effect.